(1) Field of the Invention
The present invention relates to semiconductor light-emitting devices, such as light-emitting diodes or semiconductor laser diodes.
(2) Description of Related Art
In recent years, use of a Group III-V nitride semiconductor material has come to provide emitted light in the blue-to-ultraviolet region of the spectrum. Thus, light-emitting diodes capable of emitting three primary colors, red (R), green (G) and blue (B), respectively, have all been available. Hence, all the light rays in the visible region of the spectrum have been able to be produced by light-emitting diodes. This has led to expansion of the market for light-emitting diodes serving as light sources for display applications and light sources for illumination.
A blue light-emitting diode using a Group III-V nitride semiconductor material according to a first known example will be described hereinafter with reference to FIG. 15. As illustrated in FIG. 15, a blue light-emitting diode is configured such that an n-type GaN layer 102, an active layer 103 of undoped InGaN and a p-type GaN layer 104 are sequentially formed on the principal surface of a substrate 101 of sapphire by epitaxial growth. Electrons are injected from the n-type GaN layer 102 into the active layer 103, and holes are injected from the p-type GaN layer 104 thereinto. The injected electrons and holes are recombined in the active layer 103, thereby producing emitted light rays 110 and 112.
Among the emitted light rays produced by the recombination of electrons and holes in the active layer 103, the emitted light ray 110 forming a smaller angle relative to the normal to a light-outputting surface 105 of the blue light-emitting diode than the critical angle θ1 is delivered to outside as an outgoing light ray 111. On the other hand, the emitted light ray 112 forming a larger angle relative to the above-mentioned normal than the critical angle θ1 is totally reflected so as to be confined, as an internal light 113, within the p-type GaN layer 104 and therefore is not allowed to outgo. The critical angle θ1 herein is determined by the following formula (1).θ1=sin−1(N1/N2)  (1)
Herein, N1 represents the refractive index of a surrounding of a semiconductor (air) for an emitted light ray, and N2 represents the refractive index of a semiconductor for an emitted light ray.
For example, the refractive index N2 of a GaN-based semiconductor is approximately 2.4, and the refractive index N1 of air is 1. Accordingly, the angle θ1 is approximately 25° when determined in accordance with the formula (1). The emitted light produced in the active layer 103 is radiated substantially isotropically. When the critical angle θ1 is small like approximately 25°, this prevents many emitted light rays from being extracted from the diode.
To cope with this, like a blue light-emitting diode according to a second known example illustrated in FIG. 16 (see, for example, Japanese Unexamined Patent Publication No. 2005-5679), the configuration of a blue light-emitting diode has been proposed in which a resin film formed with corrugations of a two-dimensional periodic structure is formed on a semiconductor layer to prevent the light extraction efficiency with which light can be extracted from the blue light-emitting diode from decreasing due to total reflection. More specifically, a film 106 of a polycarbonate resin material is formed on a p-type GaN layer 104. The resin film 106 is formed at its top surface with two-dimensional periodic corrugations 106a. 
Diffraction vectors effected by the two-dimensional periodic corrugations 106a allow many of emitted light rays to be incident upon the interface 107 between the p-type GaN layer 104 and the resin film 106 at a smaller angle than the critical angle θ1. This enhances the light extraction efficiency. In particular, in the use of a semiconductor material of a high etching resistance, e.g., a nitride semiconductor, after the formation of the resin film 106 on the top surface of the semiconductor material, a stamper formed at its pressing surface with corrugations is pressed against the formed resin film 106. In this manner, the two-dimensional periodic corrugations 106a are formed by transferring the corrugations of the stamper to the resin film 106.
Meanwhile, since in general a resin material has a different refractive index from a semiconductor, total reflection takes place also at the interface 107. In this case, the critical angle θ2 is expressed by the following formula (2).θ2=sin−1(N3/N2)  (2)
Herein, N3 represents the refractive index of a resin material for an emitted light ray, and N2 represents the refractive index of a semiconductor for an emitted light ray.
The refractive index N3 of a resin material is approximately 1.6, and the refractive index N2 of a GaN-based semiconductor is approximately 2.4. The critical angle θ2 is approximately 42° when determined in accordance with the formula (2). Even when like the second known example a resin film 106 having a smaller refractive index than the p-type GaN layer 104 is formed on the p-type GaN layer 104, light extraction efficiency is insufficient.